1. Field of the Invention
This invention relates to waterbed structures, and more particularly, to a constant-pressure waterbed that can provide a constant air pressure to the water-filled mattress so as to allow the water in the water-filled mattress to be always suitably pressurized to provide the desired level of softness.
2. Description of Related Art
A waterbed is a water-filled rubber or plastic mattress mounted in a surrounding air-pressure frame. Due to the highly deformable property of water, the water-filled mattress can provide a soft surface that conforms to the sleeper's body in any position, allowing a very comfortable and massaging effect for the sleeper resting thereon. The water-filled mattress can be adjusted in surface softness by varying the air pressure being applied thereto from the air-pressure frame. When the air-pressure frame is pressurized, it can compress the water-filled mattress to provide a harder surface; when the air-pressure frame is depressurized, it can cause the water-filled mattress to provide a softer surface.
From experiments and user responses, it has been learned that the most suitable air pressure from the air-pressure frame to cause the water-filled mattress to provide the most comfortable softness lies within the range from 0.1 kg/cm.sup.2 to 0.3 kg/cm.sup.2 (kilogram per square meter).
In use, however, the surface softness of the water-filled mattress can be undesirably affected in many ways due to changes in the total weight acting on the mattress. For instance, a weighty sleeper resting on the water-filled mattress will cause the mattress to be more pressurized than a light-weight sleeper. Therefore, if a first user is resting on a waterbed that has been previously adjusted in pressure for a second user, the first user may feel the mattress to be overly hard or soft to rest one if he/she is considerably more weighty or less weighty than the second user. Moreover, if two users are resting on such a waterbed, the increased total weight would undoubtedly make the waterbed much harder, and thus more uncomfortable to sleep on. In these circumstances, the waterbed needs to be readjusted in the air pressure in the air-pressure frame. If the waterbed is frequently used by different users, the repeated adjustments will be laborious and inconvenient.
There exists, therefore, a need for a waterbed whose air-pressure frame can always maintain the applied air pressure at a substantially fixed level, so as to allow the water-filled mattress to constantly provide the desired level of softness irrespective of any changes in the total weight acting on the water-filled mattress.